The present invention disclosed herein relates semiconductor device and is, for example, applicable semiconductor device equipped with a ring oscillator for detecting a delay.
As significant characteristic degradations of micro CMOS processes, there are a Bias Temperature Instability (BTI) degradation and a Dielectric Breakdown degradation. The BTI degradation is further classified into two types: Negative Bias Temperature Instability (NBTI) degradation and Positive Bias Temperature Instability (PBTI) degradation. NBTI degradation refers to a phenomenon such that, when the temperature of a chip rises in a condition that the substrate potential of a P-channel MOS transistor is back biased with respect to a gate potential, an absolute value of the transistor's threshold voltage increases gradually and the transistor speed slows down, as time passes. PBTI degradation refers to a phenomenon such that, when the temperature of a chip rises in a condition that the substrate potential of an N-channel MOS transistor is forward biased with respect to a gate potential, an absolute value of the transistor's threshold voltage increases gradually and the transistor speed slows down, as time passes.
Delay caused by, inter alfa, a characteristic degradation of MOSFETs, as described above, is detected by a ring oscillator or the like. A ring oscillator is an oscillator configured by coupling gate circuits, each producing an output by inverting its input, in a ring form with odd numbered stages. Besides a ring oscillator which is comprised of inverters as the gate circuits, usage of a ring oscillator which is comprised of NAND circuits or a ring oscillator which is comprised of NOR circuits is proposed (Japanese Unexamined Patent Application Publication No. 2012-202722).